Recently, SLDs, which may be positioned between semiconductor lasers and light emitting diodes, have drawn attention. This light emitting device has a broad width spectrum like the light emitting diode and emits light having output power as high as that of the semiconductor laser. SLDs are useful light sources for fiber-optic gyroscopes, which are installed in inertial navigation systems for detecting positions of air planes or ships at every moment and directing them to their destinations. SLDs can emit low coherent light at high output power with good directionality.
In designing and producing high power SLDs, it is important to prevent laser oscillation and to broaden the spectrum width of spontaneous emission light.
FIG. 6 is a perspective view showing a prior art nonexcitation absorption type SLD. FIG. 7 is a plan view thereof. In these figures, reference numeral 1 designates an n type GaAs substrate. An n type AlGaAs cladding layer 2 is disposed on the substrate 1. An undoped AlGaAs active layer 3 is disposed on the n type AlGaAs cladding layer 2. A p type AlGaAs cladding layer 4 is disposed on the undoped AlGaAs active layer 3. An n type GaAs cap layer 5 is disposed on the p type AlGaAs cladding layer 4. A p side electrode 6 and an n side electrode 7 are disposed on the n type GaAs cap layer 5 and the rear surface of the substrate 1, respectively. A Zn diffusion region 8 penetrates the n type GaAs cap layer 5 and reaches into the p type AlGaAs cladding layer 4. This region 8 operates to form a current injection region in the undoped AlGaAs active layer 3. As shown in FIG. 7, the Zn diffusion region 8 extends perpendicular to and from to the facet a toward and to nearly halfway between the facets a and b along the element length.
Description is given of the operation.
When a forward bias voltage is applied across the p side electrode 6 and the n side electrode 7, i.e., a positive voltage is applied to the p side electrode 6 while a negative voltage is applied to the n side electrode 7, electrons and holes are injected into the active layer 3 beneath the Zn diffusion region 8 and light-emission and recombination of the electrons and holes occur in the active layer 3, whereby spontaneous emission light and induced emission light are emitted from the facets.
FIG. 8 is a plan view of the non-excitation absorption type SLD of FIG. 7, schematically showing light generated in the excitation region c in the active layer 3. As shown by the dashed line in FIG. 8, the light C.sub.1, which is generated in the current injection region beneath the Zn diffusion region 8, i.e., the excitation region c, and has a directionality prependicular to the facet b, advances along the direction D and then it is reflected by the facet b. If the reflected light C.sub.1 returns into the excitation region c, it is combined with the light C2 having a directionality prependicular to the facet a. Then, the combined light is amplified and again reflected by the facet b and returned to the excitation region c and then further interference and amplification occur. By repeating the reflection and amplification, coherent light is generated, resulting in a laser oscillation. Once the laser oscillation occurs, it is not possible to output low power coherent light with high directionality. In order to avoid such a laser oscillation, the length l.sub.2 of the stripe-shaped excitation region c is shortened, so that the light generated in the excitation region c (including the light reflected by the facet a and amplified in the excitation region c) is absorbed in the non-excitation region d. This is called a non-excitation absorption structure. As shown by the dotted line in FIG. 8, the light A having a directionality perpendicular to the facet b is gradually absorbed by the non-excitation region d while traveling therethrough along the direction B and then it is reflected by the facet b. The reflected light is further absorbed by the non-excitation region d and does not return to the excitation region c, whereby spontaneous emission light and induced emission light are emitted from the facet. FIG. 9 shows the output spectra of the SLD shown in FIG. 8 under continuous operation at output power levels of 5 mW and 10 mW in case where the length l.sub.1 of the element is 500 microns and the length l.sub.2 and the width W of the excitation region are 250 microns and 5 microns, respectively.
FIG. 10 is a perspective view of a prior art angled-stripe type SLD disclosed in 1988, IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 24, pages 2454 to 2457, by G. A. Alphonse et al., and FIG. 11 is a plan view thereof. In FIG. 10, reference numeral 9 designates a SiO.sub.2 film.
This SLD operates in the similar way as described above. That is, when a forward bias voltage is applied across the p side electrode 6 and the n side electrode 7, spontaneous emission light and induced emission light are emitted from the facets.
In this angled-stripe type SLD, in order to prevent the laser oscillation in the excitation region inside the element thereby to obtain spontaneous emission light and induced emission light emitted from the facets, the stripe-shaped Zn diffusion region 8 is angled by 5.degree. with respect to the facets and the length of the element is twice as long as that of the SLD shown in FIG. 6, i.e., 1000 microns. FIG. 12 is a plan view of the SLD shown in FIG. 10, schematically showing light generated in the excitation region c.sub.1 in the active layer 3. For example, the light E.sub.1 advances along the excitation region c.sub.1 and is output as the light E.sub.2. At this time, although the light E.sub.3 is generated as a reflection component of the light E.sub.1, since the excitation region c is inclined by 5.degree. with respect to the facets, this reflection component E.sub.3 does not again return to the excitation region c.sub.1 but it is absorbed by the nonexcitation region d.sub.1, so that no laser oscillation occurs. In addition, since the element length l.sub.1 is as long as 1000 microns, the lights F.sub.1 and F.sub.3 having directionality perpendicular to the facets a.sub.1 and b.sub.1, respectively, which may contribute to the laser oscillation, are reflected by the facets a.sub.1 and b.sub.1 and absorbed by the non-excitation region d.sub.1 before returning into the excitation region c.sub.1, so that no laser oscillation occurs. FIGS. 14(a) to 14(c) show output spectra of the SLD shown in FIG. 12 under a continuous operation at output power levels of 8 mW, 14 mW and 28 mW, respectively, in case where the element length l.sub.1 is 1000 microns and the width W of the excitation region is 5 microns.
In the prior art non-excitation absorption type SLD constituted as described above, a broad spectrum as shown in FIG. 9 can be obtained in the low power output operation at an output power level of 5 mW. However, in the high power output operation at output power level of 10 mW, the intensity of the light generated in the excitation region due to the light-emission and recombination of carriers is increased and the ratio of the light reflected by the facet a and amplified in the excitation region c in FIG. 8 is increased, so that the light is not sufficiently absorbed by the non-excitation region and the reflection and amplification of the light having directionality perpendicular to the facets is repeated, resulting in a laser oscillation.
In the prior art angled-stripe type SLD constituted as described above, since light confinement effect of the stripe-shaped Zn diffusion region 8 in the horizontal direction is poor, modes F.sub.1 and F.sub.2, which reciprocate between the both facets as shown in FIG. 12, are likely to occur and laser oscillation occurs in high power output operation. Such a laser oscillation can be prevented by further increasing the element length. In this case, however, the increased size of the element results in poor production yield. In addition, although the width W of the Zn diffusion region 8 is 5 microns in the above description, the effective width is about 30 to 40 microns because of the diffusion current. As the result, modes H.sub.1 and H.sub.2, which reciprocate between the facets as shown in FIG. 13, are likely to occur.